1. Field of the Invention
The present invention relates to a non-aqueous secondary cell, especially a lithium secondary cell having a higher energy density and a higher output voltage for the end use.
2. Description of Related Art
Various cordless appliances, especially portable electronic appliances with semiconductor devices have desirably needed a secondary cell having a higher energy density and higher working voltage as a power supply for rendering the appliances more miniaturized and lighter in weight as the semiconductor devices have been developed to work at lower voltages. Non-aqueous electrolyte secondary cells with negative electrode active materials comprising alkali metals such as lithium and the like have been greatly expected to promise a family of cells which can satisfy the needs because they have advantages of a higher energy density and a lower weight. Therefore, research has been predominantly made to develop the lithium secondary cells. As positive electrodes for these lithium secondary cells, there have been proposed for example, oxides such as V.sub.2 O.sub.5, TiO.sub.2, and MnO.sub.2 ; sulfides such as TiS.sub.2 ; lithium-containing metal oxides comprising predominantly lithium and transition metals such as LiCoO.sub.2, LiNiO.sub.2 and the like as disclosed in Japanese Patent Publication (Post-Exam.) No. Sho 63-59507. A combination of any one of these positive electrodes with a negative electrode using lithium has made it possible to develop a non-aqueous secondary cell having a higher voltage on the order of 3 to 4 V.
However, the use of metallic lithium in the foil form as negative electrode led to an increased rate of deterioration of the negative electrode at the time of discharging and a reduced cycle life precluding charging and discharging at a high efficiency, though the theoretical energy density was increased. Moreover, it raised salty problems as the risks of ignition and burst while using the cells owing to the generation of lithium dendrite. Although research has been made to use an alloy of lithium with other metals such as aluminum and bismuth, there have also been produced problems of a reduction in the discharge voltage of the cells and unsatisfactory cycle life property.
As measures to cope with these problems, there have been proposed techniques of using lithium-containing metal oxides such as LiCoO.sub.2 as positive electrode active materials together with negative electrodes such as carbonaceous elements produced with powdery carbon such as petroleum coke and pitch coke or by a vapor phase growth process as disclosed in Japanese Patent KOKAIs (Laid-open) Nos. Sho 63-121260 and Hei 1-204361, or with shapes of graphite or carbon particles having specific physical properties as disclosed in U.S. Pat. No. 4,423,125, Japanese Patent KOKAIs (Laid-open) Nos. Sho 57-208079 and Sho 63-102166 where lithium ions are intercalated or deintercalated.
The non-aqueous electrolyte secondary cells are desired to have excellent characteristics of a high rate of discharging, longer cycle life, higher energy density per unit weight and unit volume as well as a discharge voltage of 3 V or more, preferably 3.5 to 4 V or more. However, the conventional non-aqueous electrolyte secondary cells had problems of a lower reactivity and higher internal resistance compared to the aqueous secondary cells for alkaline storage batteries, so that they were required to have an increased rate of utilization of the active materials by using thinner positive and negative electrodes of a larger surface area (longer dimension) for measures to solve the problems.
Examination of some of the pseudo-graphitic materials as described above for negative electrode properties indicated that they had a capacity of 100 to 188 mAh/g of the carbonaceous materials and relatively noble potential at the carbon electrode during charging and discharging. It has been found therefore that even a combination with such a positive electrode as the LiCoO.sub.2 having a high voltage and a high capacity resulted in a relatively low cell voltage and a low energy density.
Generally, the amount of lithium to be chemically intercalated between graphite layers has been reported to be limited at most to that corresponding to a graphite intercalation compound, C.sub.6 Li (referred to as first stage) where one lithium atom is intercalated per six carbon atoms. The capacity in this case is theoretically 372 mAh/g of the carbonaceous material, while the use of the aforementioned pseudo-graphitic materials results in a much lower capacity. This is considered attributable to a smaller amount of lithium intercalated due to insufficient growth of the graphite layer structure (lower crystallinity).
As improved materials over the aforementioned pseudo-graphitic materials, graphitic materials produced by graphitization of carbon pitch fibers at 2500.degree. C. to 2900.degree. C. as disclosed in, for example, Japanese Patent KOKAI (Laid-open) No. Hei 2-82466, and those produced by graphitization of mesophase carbon fibers spun from molten pitch which we studied exhibited a possibility of achieving a capacity of about 200 to 300 mAh/g (at initial cycle), but there remain still problems that they alone could not be formed into a high density sheet when thin and large area negative electrode sheets were to be made and that even if the carbon fibers were ground, the bulk density of the powder was so small that the volumetric energy density of the sheets made from the powder was lower.
When the graphite materials having a high crystallinity were used to produce negative electrodes, gas evolution occurred owing to the decomposition of the electrolyte at the surfaces of the graphite electrodes at charging to cause a reduction of the intercalation reaction of lithium as reported in Journal "Surface" Vol. 21, No. 1, (1983) pp. 2-13 and Japanese Patent KOKAI (Laid-open) No. Hei 2-82466.
On the other hand, graphitized materials having a relatively high crystallinity produced by heat-treating coke at high temperatures have found to be of a relatively high capacity (200 to 250 mAh/g) though accompanied by gas evolution. However, as the graphite repeated expansion and shrinkage in the direction of C axis with the reactions of intercalation and deintercalation of lithium and the magnitude of the variation was large due to the high capacity, generally the cycle characteristics of these graphitized materials having a high crystallinity were consequently less preferable owing to the swelling of the negative electrodes.
It has been found, therefore, that the graphite having a lower crystallinity is preferred in view of the cycle property while the graphite having a higher crystallinity is preferred in view of the capacity property.